Plant breeding is one of the oldest accomplishments of man. It began when plants were domesticated by growing them under controlled conditions and selecting those types that provided a dependable source of food. The most important feature contributing to high yield of many new varieties is their hybrid nature. The most dramatic example is hybrid corn, which was first introduced in significant amounts in 1932 and now makes up about 95% of the acreage of corn in the United States. Hybrid varieties are now available in crops such as sorghum, sugar beet, sunflower, onions, castor beans, oilseed rape, leek, cucumber, tomato, spinach, melon, pepper, carrot, cabbage, cauliflower, broccoli, radish, egg plant etc., in fungi, such as mushrooms, and in animals, such as poultry and fish.
J. Sneep and A. Hendriksen (1979, Pudoc, Centre for Agricultural Publishing and Documentation Wageningen), teach several methods for plant breeding which have been successfully applied during the last decades and which result in the varieties that are grown nowadays. In the Chapter “Current breeding methods”, J. Sneep and A. Hendriksen (1979) (supra, pp 104-233), describe general breeding techniques but also the specific breeding technologies for a number of crops, such as potato, sugar beet, maize, sunflower etc.
In general, selections are made from a collection of plants that can be derived from seeds from the market (commercial varieties), gene bank accessions, land races etc. From this collection, the “best” plants are selected and crossed according to the art. So traditionally, pure lines or homogenous populations are obtained by breeding.
Plant breeding has the objective to produce improved crop varieties based on the exploitation of genetic variation, which exists within the germ plasm of a plant species. Genetic variation is traditionally obtained by crossing two genetically distinct plants to create hybrid progeny. The genotype of a progeny plant is the result of the combination of the genotypes of the male and female gamete, which through fusion resulted in a zygote, from which ultimately the progeny plant developed. Gametes are formed by the gametophytic generation during the life cycle of a plant and therefore the genetic variation of the gametes is reflected in the genotypes of the gametophytes. Gametophytes differentiate from spores, which are produced by the sporophytic generation during the life cycle of the plant. Spores are produced from differentiated cells in the reproductive organs of a plant through a specialized cell division process called meiosis.
During meiosis chromosomal segregation and recombination are the processes which cause independent re-assortment and the generation of new combinations of the genetic factors of a diploid genome into a haploid genome of the gametophytes. The genotype of one progeny plant is the combination of genotypes of one male and one female gamete, which fused to form a new sporophyte. Meiosis can therefore be considered to be a pivotal process during the life cycle of any living organism to create genetic variability.
This variability is used to obtain desired plants with new properties. Often the combination of the different properties of the two parents in a hybrid is more advantageous than a homozygous (parental) plant. The production of such hybrids is however rather complicated. In the case of F1 hybrids, several putative parental lines are first made homozygous, e.g. by many generations of inbreeding and selection and subsequently they are crossed in various combinations to study their combining ability. The best combinations and their respective parental lines are subsequently retained and give rise to a commercial F1 variety.
However, the normal way of obtaining desirable hybrids is rather time consuming since homozygous parental lines have to be produced first and the desired combination of two of these homozygous parental lines has then to be selected. This process requires several generations.
Also in the case of animals, like for example farm animals, such as cattle, pigs, fish, such as salmon, and fungi, such as mushrooms, hybrids may be desirable but since animals take an even longer time to become sexually mature and reproduce it takes even longer to produce homozygous lines and select for the best combination of those to produce a hybrid. Examples of animals for which the invention may be useful include rainbow trout and aquarium fish such as zebrafish.